1. Field of the Invention
The present invention relates to a blade for a rotary wing aircraft, comprising a blade tip winglet intended to reduce the blade-vortex interaction noise in descending flight.
2. Description of Related Art
It is known that all types of aircraft in which lift is obtained by one or more rotors (conventional helicopters, autogyros, compound helicopters and tilt-rotor aircraft) generate, under certain descending-flight conditions, an intense pulsating noise known as the blade-vortex interaction noise, that is due to the blades meeting the intense blade-tip vortices emitted by these same blades during the previous revolutions of the rotor(s).
Numerous special-purpose designs and devices for the tips of the blades have been proposed in the attempt to eliminate or reduce the acoustic discomfort associated with this phenomenon, but so far with limited and not enough success. These efforts are illustrated by the inventions listed below:
(1) U.S. Pat. No. 4,046,336 "Vortex diffusion and dissipation", by J. L. Tangler (Bell-Textron), filed in 1975, PA1 (2) French Patent No. 88 12188 "Pale pour voilure tournante d'aeronef et voilure tournante comportant une telle pale [Blade for aircraft rotary wing and rotary wing comprising such a blade]", by A. Damongeot (Aerospatiale SNI), filed in 1988, PA1 (3) U.S. Pat. No. 5,199,851 "Helicopter rotor blades", by F. J. Perry & A. Brocklehurst (Westland Helicopters Ltd.), under United Kingdoms priority of 1990 (European Patent 0 482 932 A1).
Patent (1) proposes that a small straight winglet be fitted to the blade tip with a view to splitting the blade tip vortex into two weaker half-strength vortices. The short distance separating these two vortices allows them to recombine into a more spread out vortex before interacting with the next blades, yielding less abrupt pressure variations at the surface of the blades and a less pulsating noise. It has, however, been shown that, on the one hand, the excessively small size of the winglet (the chord is equal to 20% of that of the blade, and the span between 25 and 50% of the chord of the blade) probably does not allow two vortices of equal strengths to be obtained and that, on the other hand, the absence of twist in this winglet means that there is a risk that the air streams will detach at the junction with the actual blade, which will increase the drag and loss of performance.
Patent (2) proposes another principle aimed both at improving the performance and at reducing the noise: the distribution of lift along the span is modified in such a way as to very gradually reduce this lift toward the tip, and this eliminates or appreciably reduces the strength of the blade tip vortex under certain conditions, especially in hovering flight. This modification of the lift is obtained essentially by means of a local increase in the twist (more negative) according to a predefined relationship which can be adapted to suit the rectangular or otherwise shape of the tip, in plan view (chord relationship).
Patent (3), put forward as an improvement to Patent (1), again makes use of the principle of splitting the blade tip vortex, but recommends the use of a winglet of a far larger size both in the direction of the chord (equal to or greater than 50% of the chord of the blade) and in the direction of the span (between 4 and 8% of the radius of the rotor). In addition, twisting the winglet in the opposite direction to the twist in the blade is recommended so as to unload the region where the winglet and the blade meet and thus avoid any detachment. Offsetting the leading edge of the winglet forward (by the order of 15% of the chord with respect to the leading edge of the blade) is associated with a very pronounced sweeping of the winglet (an angle of 42.degree. at the leading edge) with the intention of reducing the pitching moment of the blade as boundary layer detachment approaches. Finally, Patent (3) incorporates a slight positive dihedral angle (5.degree. upward) and a change in profile along the winglet (tapering toward the end). However, there is no change in profile where the winglet meets the blade.